Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. The method for producing such glass-ceramics customarily involves three fundamental steps: first, a glass-forming batch is melted; second, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and third, the glass body is heated to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ.
Frequently, the glass body is exposed to a two-stage treatment. Hence, the glass will be heated initially to a temperature within, or somewhat above, the transformation range for a period of time sufficient to cause the development of nuclei in the glass. Thereafter, the temperature will be raised to levels approaching, or even exceeding, the softening point of the glass to cause the growth of crystals on the previously-formed nuclei. The resultant crystals are commonly more uniformly fine-grained, and the articles are typically more highly crystalline. Internal nucleation allows glass-ceramics to possess such favorable qualities as a very narrow particle size distribution and highly uniform dispersion throughout the glass host.
Transparent glass-ceramics are well known to the art; the classic study thereof being authored by G. H. Beall and D. A. Duke in "Transparent Glass-Ceramics", Journal of Materials Science, 4, pp. 340-352 (1969). Glass-ceramic bodies will display transparency to the human eye when the crystals present therein are considerably smaller than the wavelength of visible light. More specifically, transparency generally results from crystals less than 50 nm, and preferably as low as 10 nm in size.
Recently, much effort has been concentrated in the area of using transparent glass-ceramics as hosts for transition metals which act as optically active dopants. Suitable glass-ceramic hosts must be tailored such that transition elements will preferentially partition into the crystals. Co-pending application Ser. No. 60/160,053, entitled "Transition Metal Glass-Ceramics" by Beall et al. is co-assigned to the present assignee, and is herein incorporated by reference in its entirety It is directed to transition-metal doped glass-ceramics suitable for formation of a telecommunications gain or pump laser fiber.
Transparent glass-ceramics which contain relatively small numbers of crystals can be of great use in cases where the parent glass provides an easy-to-melt or an-easy-to-form vehicle for a crystal. The crystal, in itself, may be difficult or expensive to synthesize, but may provide highly desirable features, such as optical activity. The crystals in the glass-ceramic are generally oriented randomly throughout the bulk of the glass, unlike a single crystal which has a specific orientation. Random orientation, and consequent anisotropy, are advantageous for many applications, one example being that of optical amplifiers, where polarization-independent gain is imperative.
Transparent glass-ceramics doped with transition elements can combine the optical efficiency of crystals with the forming flexibility of glass. For example, both bulk (planar) and fiber forms can be fabricated from these glass-ceramics.
Therefore, there exists a need for transparent glass-ceramic materials which contain small tetrahedral and interstitial sites, and hence are suitable as potentially valuable hosts for small, optically active transition elements. Such elements include, but are not limited to, Cr.sup.4+, Cr.sup.3+, Co.sup.3+, Co.sup.2+, Cu.sup.2+, Mn.sup.2+, Cu.sup.2+, and Ni.sup.2+. These elements impart luminescence and fluorescence to such doped, glass-ceramic materials, thereby rendering them suitable for application in the optical field industry.
The crystal structures of both alpha- and beta-willemite (i.e., zinc orthosilicate (Zn.sub.2 SiO.sub.4)) consist of frameworks of SiO.sub.4 and ZnO.sub.4 tetrahedra.
The alpha-willemite structure was determined in 1930. It is isostructural with phenacite (Be.sub.2 SiO.sub.4), with rhombohedral space group R 3, and consists of linked SiO.sub.4 and ZnO.sub.4 tetrahedra. All Zn.sup.2+ ions occur in tetrahedral coordination. Each oxygen atom is linked to one silicon and two zinc atoms.
The beta-willemite phase has a crystal structure related to those of the silica polymorphs tridymite and cristobalite. Half of the zinc ions are in tetrahedral coordination while the remaining half lie in interstitial positions. Phase equilibrium studies confirm that the alpha-willemite form is the sole thermodynamically stable binary compound in the ZnO--SiO.sub.2 system. However, the metastable beta-willemite is obtained quite readily as a devitrification product in glasses. When held at temperatures above 850.degree. C., beta-willemite ultimately transforms to the stable alpha polymorph.
The beta-willemite phase offers several potentially useful properties. Unlike alpha-willemite, beta-willemite can have a widely variable composition, ranging from 33 to 67 mole % ZnO. This wide range of solid solution allows the phase to be obtained in glass-ceramics of widely varying composition.
Glass-ceramics containing the alpha-willemite form of Zn.sub.2 SiO.sub.4 are known, particularly as materials for electronic applications. U.S. Pat. No. 4,714,687 is directed to glass-ceramic materials containing willemite as a predominant crystal phase and especially designed for substrates in integrated circuit packaging. The glass-ceramic consists essentially, in terms of weight percent, of 30-55 SiO.sub.2, 10-30 Al.sub.2 O.sub.3, 15-45 ZnO, and 3-15 MgO.
However, what the prior art has failed to disclose, and what this invention teaches, is a willemite glass-ceramic material that is transparent and is suitable for employment in the fiber optic industry.
Accordingly, the primary object of the present invention is to provide glass-ceramic materials which are substantially and desirably totally transparent, and which contain a predominant willemite crystal phase.
Another object of the present invention is to provide such willemite glass-ceramics which are capable of being doped with ingredients that confer luminescence and/or fluorescence thereto.
An important advantage of the present glass-ceramic family is that it provides a material containing a willemite crystalline phase which can be tetrahedrally-coordinated with transition metal ions including, but not limited to, Cr.sup.4+, Cr.sup.3+, Co.sup.3+, Co.sup.2+, Cu.sup.2+, Mn.sup.2+, Cu.sup.2+, and Ni.sup.2+. Further, the glass-based thus providing the important flexibility of allowing for fabrication of both bulk (such as planar substrates) and fiber (such as optical fiber) forms.
Other objects and advantages of the present invention will be apparent from the following description.